When a well is completed in a subterranean reservoir, at least some of the oil present in the reservoir is removed from the reservoir through the well by primary recovery methods. These methods include utilizing native reservoir energy in the form of water or gas existing under sufficient pressure to drive the oil from the reservoir through the well to the earth's surface. This native reservoir energy most often is depleted long before all of the oil present in the reservoir has been removed from it. Additional oil can be removed by enhanced recovery methods comprising adding energy from outside sources to the reservoir either before or subsequent to the depletion of the native reservoir energy.
Such enhanced recovery methods include miscible phase displacement techniques wherein a fluid or fluids miscible with the reservoir oil are introduced into the reservoir through an injection well to displace the oil from the pores of the reservoir and drive it to a production well. The miscible fluid is introduced into the injection well at a sufficiently high pressure to drive the body of fluid through the reservoir where it collects and drives the reservoir oil to the production well.
The process of miscible flooding is extremely effective in stripping and displacing the reservoir oil from the reservoir through which the solvent flows. The high degree of effectiveness is derived from the fact that a two-phase system within the reservoir and between the solvent and the reservoir is eliminated at the conditions of temperature and pressure of the reservoir, thereby eliminating the retentive forces of capillarity and interfacial tension which are significant factors in reducing the recovery efficiency of oil in conventional flooding operations where the displacing agent and the reservoir oil exist as two phases in the reservoir.
More recently, various gases, such as nitrogen, carbon dioxide (CO.sub.2) and methane, have been used successfully as displacing fluids in oil recovery processes. Carbon dioxide is a particularly desirable material because it is highly soluble in oil. The dissolution of carbon dioxide in oil decreases the oil viscosity and increases the volume of oil, both of which improve the recovery efficiency of the process. Carbon dioxide is sometimes employed under immiscible conditions. However, the most efficient displacement of reservoir oil by CO.sub.2 occurs when the CO.sub.2 is miscible in the oil. This miscibility takes place at a pressure greater than a certain minimum, see Stalkup, F.I., "Carbon Dioxide Miscible Flooding: Past, Present, and Outlook for the Future", Journal of Petroleum Technology, (August 1978) pp. 1102-1112. This minimum pressure is defined as the carbon dioxide minimum miscibility pressure (MMP). The magnitude of the MMP depends on the properties and composition of reservoir oil and the purity of the CO.sub.2 and temperature. The effect of temperature is discussed in an article by Yellig et al, "Determination and Prediction of CO.sub.2 Minimum Miscibility Pressures", Journal of Petroleum Technology, (1980), Vol. 32, pp. 160-168, where it is shown that for every 50.degree. F. drop in temperature, the CO.sub.2 MMP decreases by about 600-700 psia.
It is also known to use tall oil as an additive in aqueous surfactant systems, e.g., see Chin, U.S. Pat. Nos., 3,892,668 and 3,823,774, Reisberg, U.S. Pat. No. 3,330,344, Williams et al, U.S. Pat. No. 3,497,007 and Chiu, U.S. Pat. No. 3,943,059.